This invention relates to a pH indicator composition having controlled change of color at a selected sub-freezing temperature. This invention also relates to a method of making and using the composition. Additionally, this invention relates to an article of manufacture comprising a casing containing the pH indicator composition of this invention. In a preferred embodiment, the article of manufacture is a color change thermometer.
It is generally recognized that pH indicators consist of an acid-base couple in which the acid has a color different from that of the base. These pH indicators are generally used to determine or at least give an idea of the pH value of a solution. A limitation of such pH indicators is that they cannot give an exact determination of the pH value, since they have a transition interval for the color change from acid to base form which amounts to about 1 to 2 pH units. The transition interval is also referred to by workers in the art as the indicator range or the color change interval, and is recognized as the range of pH values within which pronounced color change takes place. This transition interval is one reason why in recent years pH indicators have increasingly been replaced by electrical pH meters, which are capable of measuring the pH value of a solution more accurately. A still more serious limitation, which is common to both pH indicators and pH meters, is that they do not make it possible to measure the pH values of frozen solutions, i.e. substances that are in a solid state. The reason why pH indicators usually cannot be used to indicate the pH of a frozen solution is that the color of the pH indicator is not stable when the solution freezes. Rather, the color is prone to change.
Another problem which seemingly is not associated with the above-mentioned problem is that there has been no simple and efficient device available hitherto to permit visual indication, in a readily perceivable fashion, of temperature conditions, time-temperature conditions or temperature-space conditions, e.g. temperature distribution in a freezer compartment. For instance, in order to record the temperature in refrigerated boxes, freezers or freezer storage compartments, use has been made of conventional thermometers or temperature sensitive electrical means. These known devices for measuring the temperature, however, suffer from the drawback of sensing the temperature only at a single point or location in the compartment concerned. Consequently, a general picture of the temperature conditions prevailing in the compartment is not obtained. Moreover, the picture obtained may be misleading if the sensing means, as is often the case, is disposed in the coldest part of the compartment, such as adjacent to the supply opening for cooling air in a freezer compartment.
Thus, there has been a need in the art for a solution to the aforementioned problems. Specifically, there has been a need for a pH indicator composition, method of making the composition and method of using the composition applicable to frozen solutions. There has also been a need in the art for a method and means of measuring sub-freezing temperatures and for measuring time-temperature and temperature-space conditions, such as in a freezer compartment.